1. Field of the Invention
The present invention relates to a narrow-angle directional microphone and a method of manufacturing the same, and specifically relates to a narrow-angle directional microphone using an acoustic tube and a method of manufacturing the same.
2. Description of the Related Art
In narrow-angle directional microphones having an acoustic tube, a front acoustic terminal of a microphone unit and a rear end of the acoustic tube are connected and a connected portion is sealed so that external sound waves do not enter through the connection portion. Such a configuration realizes significantly narrow directivity. However, there are drawbacks of easily catching external wind noise and a high proximity effect to distort sound in a low frequency range when a sound source is close.
As a configuration to solve the above-described drawbacks, the applicant of the present application discloses, in Japanese Patent No. 2562295, a configuration to provide a plurality of openings (sound wave introduction ports) in a tube wall of an acoustic tube (aluminum tube) that accommodates a microphone unit, and to stick acoustic resistances (fabric, non-woven fabric, or the like) to outside portions of the openings.
The configuration disclosed in Japanese Patent No. 2562295 can have lower wind noise and proximity effect than conventional narrow-angle directional microphones.
By the way, as illustrated in FIG. 8, an acoustic tube 50 in which an acoustic resistance material 52 is stuck to openings 51 is covered with a microphone case 55 having openings 56 outside to mechanically protect the acoustic tube 50 and form electrostatic shield. However, typically, the microphone case 55 is also formed of aluminum, and thus there is a problem of an increase in weight as the entire narrow-angle directional microphone.
As a method to solve the problem, use of the microphone case as the acoustic tube can be considered. In that case, the microphone case is provided with a plurality of acoustic passage openings, and a mesh-like acoustic resistance material made of a sheet-like conductive material is stuck to an inside of the case. A configuration to block the openings with the acoustic resistance material is used. The reason to form the acoustic resistance material with the conductive material is to configure the electrostatic shield. Accordingly, the microphone case also serves as the acoustic tube, and thus the weight as the entire microphone becomes lighter.
However, an adhesive is applied to an inside surface of the microphone case, and the sheet-like acoustic resistance material is stuck. Therefore, there is a problem that difficulty in processing is high. Further, there is a problem that acoustic characteristics are deteriorated when contact failure occurs.
To solve the above-described problems, as a result of diligent examination, the applicant of the present application has found a method for allowing the acoustic resistance to be closely attached to the opening of the microphone case with a force of a cylinder to develop, which cylinder is prepared by rounding an elastic acoustic resistance sheet and inserted into the microphone case to inscribe thereto. In this case, as illustrated in FIG. 9 (sectional view), a sheet-like acoustic resistance material 61 is accommodated in a microphone case 62 in a state of being cylindrically rounded such that both right and left ends of the sheet-like acoustic resistance material 61 are in contact.
However, there is a small difference between a diameter of the cylindrical acoustic resistance material 61 and an inner diameter of the microphone case 62, and thus the cross section of the cylindrical acoustic resistance material 61 is not a perfect circle. Therefore, even if opening portions 63 of the microphone case 62 can be covered with the acoustic resistance material 61, there is a possibility that gaps 70 (thin air layers) are formed between the acoustic resistance material 61 and inner surface of the microphone case 62. If there are such gaps, an acoustic resistance value is decreased (the gaps are equivalent to a case where the openings of the microphone case become larger). Therefore, there is a problem that sound waves inside the acoustic tube leak.